Ethylene production in plants and plant parts is induced by a variety of external factors and stressors, including wounding, the application of hormones (e.g., auxin), anaerobic conditions, chilling, heat, drought, and pathogen infection. Increased ethylene production also is observed during a variety of plant development processes, including fruit or vegetable ripening, seed germination, leaf abscission, and flower senescence.
Ethylene biosynthesis in plants is typically depicted as an enzymatic scheme involving three enzymes, traditionally referred to as the “Yang Cycle,” in which S-adenosyl-L-methionine (SAM) synthase catalyzes conversion of methionine to S-adenosyl-L-methionine (AdoMet); 1-aminocyclopropane-1-carboxylic acid (ACC) synthase catalyzes the conversion of AdoMet to ACC; and ACC oxidase catalyzes the conversion of ACC to ethylene and the byproducts carbon dioxide and hydrogen cyanide. See, for example, Srivastava (2001) Plant Growth and Development: Hormones and Environment (Academic Press, New York) for a general description of ethylene biosynthesis in plants and plant development processes regulated by ethylene.
Previous research has established that in climacteric fruit ripening is triggered, at least in part, by a sudden and significant increase in ethylene biosynthesis. Although a sudden burst of ethylene production is implicated in the fruit ripening process of climacteric fruits, the exact mechanism, particularly in nonclimacteric fruits, is not completely understood. While there is no sudden burst of ethylene production in non-climacteric fruit, non-climacteric fruit will respond to ethylene. Moreover, fruits, vegetables, and other plant products vary in the amount of ethylene synthesized and also in the sensitivity of the particular product to ethylene. For example, apples exhibit a high level of ethylene production and ethylene sensitivity, whereas artichokes display a low level of ethylene biosynthesis and ethylene sensitivity. See, for example, Cantwell (2001) “Properties and Recommended Conditions for Storage of Fresh Fruits and Vegetables” at postharvest.ucdavis.edu/Produce/Storage/index.shtml (last accessed on Mar. 6, 2007), which is herein incorporated by reference in its entirety. Fruit ripening typically results in a change in color, softening of the pericarp, and changes in the sugar content and flavor of the fruit. While ripening initially makes fruit more edible and attractive to eat, the process eventually leads to degradation and deterioration of fruit quality, making it unacceptable for consumption, leading to significant commercial monetary losses. Control of the ripening process is desirable for improving shelf-life and extending the time available for transportation, storage, and sale of fruit and other agricultural products subject to ripening.
In addition to a sudden increase in ethylene biosynthesis in climacteric fruits, ripening-related changes are also associated with a rise in respiration rate. Heat is produced as a consequence of respiration in fruit, vegetables, and other plant products and, consequently, impacts the shelf-life and the required storage conditions (e.g., refrigeration) for these commodities. Plant products with higher rates of respiration (e.g., artichokes, cut flowers, asparagus, broccoli, spinach, etc.) exhibit shorter shelf-lives than those with lower respiration rates (e.g., nuts, dates, apples, citrus fruits, grapes, etc.). Respiration is affected by a number of environmental factors including temperature, atmospheric composition, physical stress, light, chemical stress, radiation, water stress, growth regulators, and pathogen attack. In particular, temperature plays a significant role in respiration rate. For a general description of respiratory metabolism and recommended controlled atmospheric conditions for fruits, vegetables, and other plant products see, for example, Kader (2001) Postharvest Horticulture Series No. 22A:29-70 (University of California—Davis); Saltveit (University of California—Davis) “Respiratory Metabolism” at usna.usda.gov/hb66/019respiration.pdf (last accessed on Mar. 6, 2007); and Cantwell (2001) “Properties and Recommended Conditions for Storage of Fresh Fruits and Vegetables” at postharvest.ucdavis.edu/Produce/Storage/index.shtml (last accessed on. Mar. 6, 2007), all of which are herein incorporated by reference in their entirety.
Methods and compositions for delaying the fruit ripening process include, for example, the application of silver salts (e.g., silver thiosulfate), 2,5-norbornadiene, potassium permanganate, 1-methylcyclopropene (1-MCP), cyclopropene (CP) and derivatives thereof. These compounds have significant disadvantages, such as the presence of heavy metals, foul odors, and explosive properties when compressed, that make them unacceptable for or of limited applicability for use in the food industry. Transgenic approaches for controlling ethylene production to delay plant development processes (e.g., fruit ripening) by introducing nucleic acid sequences that limit ethylene production, particularly by reducing the expression of the enzymes ACC synthase or ACC oxidase, are also under investigation. The public's response to genetically modified agricultural products, however, has not been entirely favorable.
Accordingly, a significant need remains in the art for safe methods and apparatuses to delay plant development processes. Such methods and apparatuses could provide better control of fruit ripening, vegetable ripening, flower senescence, leaf abscission, and seed germination and extend the shelf-life of various agricultural products (e.g., fruit, vegetables, and cut flowers), thereby permitting longer distance transportation of these products without the need for refrigeration, increasing product desirability to consumers, and decreasing monetary costs associated with product loss due to untimely ripening and senescence.